refs #644 Add method CCached::save, like setAndSave but using the currently set value.

Implemented with a flag in CValueCachePacket to indicate a request to save only.

src/blackmisc/datacache.h

@@ -268,6 +268,9 @@ namespace BlackMisc
 
         //! Data cache doesn't support setAndSave (because set() already causes save anyway).
         CStatusMessage setAndSave(const typename Trait::type &value, qint64 timestamp = 0) = delete;
+
+        //! Data cache doesn't support save (because currently set value is saved already).
+        CStatusMessage save() = delete;
     };
 
     /*!